Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular CMOS image sensors (“CIS”), has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of the image sensor. Thus, the number of pixels in the pixel array of the image sensor has increased, while the size of each pixel cell has decreased.
Typically each pixel of an image sensor includes a photosensitive element such as a photodiode, and one or more transistors for reading out the signal from the photosensitive element. As pixel cell size decreases, transistor sizes may decrease as well. A transfer transistor is commonly used in a pixel with a four-transistor design. The transfer transistor separates the photosensitive element from the rest of the pixel. The transfer transistor is formed between the photosensitive element and a floating node and it is desirable to scale down the transfer transistor to have a short gate length for reasons of greater integration and enhanced pixel fill factor.
In most image sensors, the constituent elements of each pixel are formed on or near what is considered the front surface of a silicon substrate, and the light to be captured by the pixels is incident on the front surface. Some image sensors known as backside-illuminated (BSI) image sensors can capture light incident on the back surface of the substrate instead of, or in addition to, capturing light incident on the front side of the substrate. In BSI image sensors, backside illumination causes the majority of photon absorption to occur near the backside silicon surface. To separate the electron-hole pairs created by photon absorption and drive the electrons toward a photosensitive region, an electric field near the back silicon surface is helpful. This electric field can be created by doping the back surface of the silicon. The quality of the doped back surface plays an important role in image sensor performance. Crystal defects and inactive dopants in the doped back surface region can degrade quantum efficiency by trapping electrons and not allowing them to reach the photosensitive region, which can result in “hot pixel” defects.
One of the major sources of crystal defects in CMOS image sensors is a result of the conventional ion implantation process, which involves dopant implantation followed by a thermal anneal to activate the implanted dopants. Thermal laser anneal is one method used to reduce the occurrence of crystal defects after ion implantation, but laser annealing creates localized heating, which can cause a significant increase in substrate temperature for a BSI CIS, since in these types of image sensors, the epitaxial (epi) layer in which the pixel is primarily formed is thin (e.g., <4 μm thick). An increase in substrate temperature can result in unintended dopant diffusion and/or metal deterioration/melting. The potential for excess heating of unintended regions may be reduced by using a thicker final epi layer, which can result by removing less of the epi layer during the substrate thinning process. However, increasing the thickness of the epi layer results in an increase in electrical crosstalk between adjacent pixels in an image sensor.
In addition to creating an increase in substrate temperature, laser anneal can also fail to activate all the backside dopant, which can result in inactive-dopant defects. These problems associated with the current fabrication process, which employs ion implantation and laser thermal annealing, can cause undesirable problems in the resulting image sensors, such as high dark current and high white pixel count.